The invention relates to a terminal station for telecommunication by wire, comprising at least two connection terminals for connecting the terminal station to a transmission line of a telecommunication network, and also comprising an anti-sidetone circuit coupled to the connection terminals.
A terminal station of this kind is generally known as an electronic telephone as described in the Philips Data Handbook "ICs for Telecom Subscriber Sets, Cordless Telephones, Mobile/Cellular Radio Pagers CA 3089 to PC 4413", Data Handbooks IC03a and IC03b, 1991.
Page 851 of the handbook IC03a shows a block diagram of such a terminal station, comprising connection terminals a/b, b/a for connecting the terminal station to a telephone line. The terminal station comprises a transmission circuit, for example as realised in the IC type 1067, and a control device in the form of a microcontroller designed for telephony purposes, for example the IC type PCD 3349. Also shown are a keypad coupled to the microcontroller and a microphone and telephone audio output device such as an earpiece or loudspeaker coupled to the transmission circuit.
FIG. 5 on page 1465 of the handbook IC03b shows an anti-sidetone circuit, forming part of the transmission circuit, in the form of a bridge circuit. This bridge circuit, including the line impedance, comprises a balance impedance and two discrete impedances which are included in the branches of the bridge. As is described on page 1464, the line impedance varies substantially as a function of the type and the length of the line. To this end, a value which is optimum for the mean line length of the transmission line is usually chosen for the balance impedance, and the further discrete impedances also have permanently adjusted values.
Integration of these components on a telephone set IC is not very feasible because of the high values of the real and the complex parts of these impedances. Moreover, generally speaking, accurate impedances in an absolute sense cannot be realised on an IC. A further problem is that different countries impose different requirements regarding the line impedance. The prior art solution to this problem has involved using external components connected to the IC.